Nitric oxide (NO) is used in numerous commercial and industrial applications. As a raw material it is used in the semiconductor industry for surface oxidation processes. As a synthesis gas, NO is used in the preparation of nitric acid, hydroxylamine, nitrosyl chloride, metal nitrosyls, and caprolactam, which is used in the synthesis of nylon. Nitric oxide is also used commercially as a polymerization inhibitor during the preparation of olefins and to modify the properties of various polymers.
Several processes have been developed for the preparation of nitric oxide. Commercially, NO is produced by the Ostwald process in which ammonia is oxidized at temperatures near 800° C. in the presence of a platinum group catalyst. Nitric oxide can also be produced from the reaction of nitric acid and copper or the reaction of sodium nitrate and sulfuric acid. These methods are not convenient for small scale NO production due to the power requirements for heating the reaction mixtures to several hundred degrees Celsius and the hazards inherent in handling strong acids. Several processes have been proposed for bench-scale production of nitric oxide for on-site use in laboratories, production facilities and medical facilities. Attention is directed to U.S. Pat. Nos. 3,853,790; 3,948,610; 4,272,336; 4,774,069; 4,812,300; 5,396,882; 5,478,549; 5,670,127; 5,683,668; 5,692,495; 5,827,420; 6,103,275; 6,534,029; 6,743,404; 6,758,214; 7,025,869; 7,040,313 and 7,048,951. Each of these methods has disadvantages relative to the photolysis of nitrous oxide (N2O). For example, some require toxic starting materials or produce toxic byproducts such as nitrogen dioxide (NO2), while others require high temperatures, high voltages or use of strong acids.
In the analysis of air and other gases for nitric oxide, it is necessary to calibrate the analytical instrument using a gas standard having a known concentration of NO. The most common method used for NO detection is based on chemiluminescence in the reaction of NO with an excess of ozone. The method, which is widely used for air pollution monitoring, for measurements of NO in automobile exhaust and for measurements of NO in exhaled breath, requires frequent calibration with a standard gas mixture. Nitric oxide measurements based on electrochemical techniques, chemiluminescence with luminol and other methods require calibration using a gas standard as well.
A well known problem with NO gas standards is that NO is unstable in gas cylinders at low concentrations; when NO standards are prepared at part-per-billion by volume (ppbv) levels there is a strong tendency for the concentration of NO in the cylinder to decline with time even though the NO is diluted into an unreactive gas such as nitrogen. One reason for this is that NO is thermodynamically unstable with respect to disproportionation to form N2O and NO2 according to the equilibrium:3NO═N2O+NO2  (3)Although extremely slow in the gas phase, this reaction may be catalyzed on the interior walls of compressed gas cylinders. The walls may be treated in various ways to slow the reaction, but the treatment is not always effective, and one cannot be certain that the concentration of NO in a gas cylinder is what it was when the cylinder was first filled. Furthermore, even trace amounts of oxygen (O2) in the diluent gas can react to oxidize NO to NO2 according to the well known reaction:2NO+O2→2NO2  (4)Also, because of reaction 4, NO compressed gas standards cannot be made with air as the diluent. This is a disadvantage since it is desirable to calibrate an NO instrument using the same diluent gas as the gas being analyzed, which is most commonly air.
Nitric oxide standards are much more stable at high concentrations of NO; thus, it is common to prepare gas standards at the high ppmv level in an unreactive gas such as N2 to make a compressed gas standard and then dynamically dilute that standard with N2 or air prior to entering the analytical instrument being calibrated. Although the dynamic dilution method works well for calibration, flow meters are required, and the flow meters must be accurately calibrated, thus adding to the complexity, expense and uncertainty of the calibration procedure.
Nitric oxide has several medical applications. Blood vessels use nitric oxide to signal the surrounding smooth muscle to relax, thus dilating the artery and increasing blood flow. This underlies the action of nitroglycerin, amyl nitrate and other nitrate derivatives in the treatment of heart disease; the compounds undergo reactions that release nitric oxide, which in turn dilates the blood vessels around the heart, thereby increasing its blood supply.
Some disorders or physiological conditions can be mediated by inhalation of nitric oxide. Dilation of pulmonary vessels in the lungs due to inhaled NO causes pulmonary gas exchange to be improved and pulmonary blood flow to be increased. The administration of low concentrations of inhaled nitric oxide can prevent, reverse, or limit the progression of disorders such as acute pulmonary vasoconstriction, adult respiratory distress syndrome, acute pulmonary edema, acute mountain sickness, post cardiac surgery acute pulmonary hypertension, persistent pulmonary hypertension in a newborn, perinatal aspiration syndrome, and asthma. For inhalation therapy, it is important that the NO gas mixture be free of the toxic gas NO2 which can form inside compressed gas cylinders.
The present invention provides a simple method for the production of nitric oxide from a non-hazardous, gas-phase precursor. Without the need for high temperatures, strong acids, and aqueous solution, the invention allows NO to be produced from a small apparatus for portable, on-site use. The concentration of NO produced can be accurately controlled, thereby making the NO source highly useful as a calibration device for analytical instruments that measure nitric oxide in gases.
The foregoing example of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.